Abstract
A total of 61,688 Hereford and 22,333 Aberdeen Angus records were studied to estimate year, year–herd, sire-within-herd, year–sire-within-herd and error variance components for weaning weight, preweaning average daily gain (ADG), yearling weight and postweaning ADG. Herd differences accounted for 25–44% of total variation for the growth traits evaluated. Estimates of year components were negligible, 4% or less, for all traits. Year–herd components accounted for 0–10% of the total variation for weaning weight and preweaning ADG, 7–14% for yearling weight and 18–25% for postweaning ADG. Sire components were generally smaller, 3–10%, than corresponding year–herd components, particularly for postweaning growth traits. Examination of the year–sire component estimates suggested a small but real year × sire interaction effect on preweaning growth. Similar effects on postweaning growth were not discerned. Error variances accounted for 40–58% of the total variation for the growth traits evaluated. Average heritabilities of weaning weight, preweaning ADG, yearling weight and postweaning ADG were.32,.32,.48 and.41, respectively.
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